Circuit design for the latest electronic equipment, including TVs, radios, computers, medical devices, office equipment and telecommunications devices, has become increasingly complex. For example, integrated circuits which contain the equivalent of several hundreds of thousands of transistors are now manufactured for these and other kinds of equipment. This rise in design complexity has been accompanied by a parallel trend toward the fabrication of ever smaller electronic components. That is, manufacturers are finding ways to fit larger numbers of such components on steadily shrinking device footprints while at the same time continuing to reduce the dimensions of the device.
Because heat generated by the various working components causes device malfunctions and inoperability, methods for effectively dissipating heat generated by electronic components are needed.
Such problems associated with heat generation are exacerbated by the increasing levels of integration being achieved in electronic components—particularly central processing units (CPUs), drivers, integrated circuits (ICs), memories and other large-scale integration (LSI) devices—which are used in electronic equipment such as personal computers (PCs), digital video disks (DVDs) and cell phones. Another current trend is a shift toward higher operating frequencies for higher performance. This has led to the generation of harmful electromagnetic waves which can cause failure, malfunction or inoperability due to electromagnetic interference between electronic components, and may have deleterious effects on the human body.
Many heat dissipating methods, as well as heat dissipating articles and compositions used in such methods, have already been devised to reduce the heat generated by electronic components. Heat sinks in the form of plates made of brass and other high thermal conductivity metals are used in electronic equipment to hold down the rise in temperature of electronic components therein during use. These heat sinks carry away heat generated by the electronic components and release that heat from surfaces by means of a temperature difference with outside air.
For heat generated by an electronic component to be efficiently transferred to a heat sink, it is necessary that the heat sink be placed in close contact with the electronic component. Because of height differences among various electronic components and component tolerances in the assembly process, a flexible heat conductive sheet or a heat conductive grease is often placed between the electronic components and the heat sink so that heat transfer from the electronic components to the heat sink takes place through the heat conductive sheet or grease. Heat conductive sheets made of materials such as heat conductive silicone rubber are used for this purpose, but a problem with such sheets is their interfacial thermal resistance.
Methods that have been proposed for lowering the interfacial thermal resistance include the use of heat conductive greases and thermosoftening sheets such as those described in JP-A 2000-509209. However, these prior-art greases and sheets serve only as heat dissipating articles, and lack the ability to absorb electromagnetic waves.
Many attempts have been made to shield out electromagnetic waves generated from electronic components. Such efforts have generally involved the use of metals, platings or electrically conductive compositions, but these materials all rely on the ability to reflect electromagnetic waves. Sheets composed of an organic rubber medium, especially chlorinated polyethylene, which is loaded with a soft magnetic powder or ferrite as the electromagnetic wave absorbing constituent are already available on the market. Yet, such sheets, while having an electromagnetic wave shielding ability, are ineffective for heat dissipation.
Materials endowed with both the ability to conduct heat and the ability to absorb electromagnetic waves have recently been described in the art. For example, JP-A 11-335472 discloses that sheet-like structures fabricated from a matrix material such as silicone gel that contains a ferrite (e.g., Mn—Zn ferrite, Ni—Zn ferrite) have electromagnetic noise suppressing effects. However, because such sheets are loaded with an electromagnetic wave absorbing filler, they are rigid. Moreover, they have a low thermal conductivity and are thus poorly suited for use as heat dissipating articles.
One object of the present invention is to provide electromagnetic wave absorbing heat conductive compositions endowed with both an excellent ability to dissipate heat and outstanding electromagnetic wave absorbing properties which suppress the generation of electromagnetic noise. Another object of the invention is to provide thermosoftening electromagnetic wave absorbing heat dissipating sheets formed from such compositions. An additional object is to provide a method of installing such compositions for the purpose of heat dissipation.